Generally, the invention relates to the field of compounds, especially peptides and polypeptides, that have thrombopoietic activity. The compounds of the invention may be used to increase production of platelets or platelet precursors (e.g., megakaryocytes) in a mammal.
This invention relates to compounds, especially peptides, that have the ability to stimulate in vitro and in vivo production of platelets and their precursor cells such as megakaryocytes. Before discussing the nature of the inventive compounds, the following is provided as a background regarding two proteins that have thrombopoietic activity: thrombopoietin (TPO) and megakaryocyte growth and development factor (MGDF).
The cloning of endogenous thrombopoietin (TPO) (Lok et al., Nature 369:568-571 (1994); Bartley et al., Cell 77:1117-1124 (1994); Kuter et al., Proc. Natl. Acad. Sci. USA 91:11104-11108 (1994); de Sauvage et al., Nature 369:533-538 (1994); Kato et al., Journal of Biochemistry 119:229-236 (1995), Chang et al., Journal of Biological Chemistry 270:511-514 (1995)) has rapidly increased our understanding of megakaryopoiesis (megakaryocyte production) and thrombopoiesis (platelet production).
Endogenous human TPO, a 60 to 70 kDa glycosylated protein primarily produced in the liver and kidney, consists of 332 amino acids (Bartley et al., Cell 77:1117-1124 (1994); Chang et al., Journal of Biological Chemistry 270:511-514 (1995)). The protein is highly conserved between different species, and has 23% homology with human erythropoietin (Gurney et al., Blood 85:981-988 (1995)) in the amino terminus (amino acids 1 to 172) (Bartley et al., Cell 77:1117-1124 (1994)). Endogenous TPO has been shown to possess all of the characteristics of the key biological regulator of thrombopoiesis. Its in vitro actions include specific induction of megakaryocyte colonies from both purified murine hematopoietic stem cells (Zeigler et al., Blood 84:4045-4052 (1994)) and human CD34+ cells (Lok et al., Nature 369:568-571 (1994); Rasko et al., Stem Cells 15:33-42 (1997)), the generation of megakaryocytes with increased ploidy (Broudy et al., Blood 85:402-413 (1995)), and the induction of terminal megakaryocyte maturation and platelet production (Zeigler et al., Blood 84:4045-4052 (1994); Choi et al., Blood 85:402-413 (1995)). Conversely, synthetic antisense oligodeoxynucleotides to the TPO receptor (c-MP1) significantly inhibit the colony-forming ability of megakaryocyte progenitors (Methia et al., Blood 82:1395-1401 (1993)). Moreover, c-MP1 knock-out mice are severely thrombocytopenic and deficient in megakaryocytes (Alexander et al., Blood 87:2162-2170 (1996)).
Recombinant human MGDF (rHuMGDF, Amgen Inc., Thousand Oaks, Calif.) is another thrombopoietic polypeptide related to TPO. It is produced using E. coli transformed with a plasmid containing cDNA encoding a truncated protein encompassing the amino-terminal receptor-binding domain of human TPO (Ulich et al., Blood 86:971-976 (1995)). The polypeptide is extracted, refolded, and purified, and a poly[ethylene glycol] (PEG) moiety is covalently attached to the amino terminus. The resulting molecule is referred to herein as PEG-rHuMGDF or MGDF for short.
Various studies using animal models (Ulich, T. R. et al., Blood 86:971-976 (1995); Hokom, M. M. et al., Blood 86:4486-4492 (1995)) have clearly demonstrated the therapeutic efficacies of TPO and MGDF in bone marrow transplantation and in the treatment of thrombocytopenia, a condition that often results from chemotherapy or radiation therapy. Preliminary data in humans have confirmed the utility of MGDF in elevating platelet counts in various settings. (Basser et al., Lancet 348:1279-81 (1996); Kato et al., Journal of Biochemistry 119:229-236 (1995); Ulich et al., Blood 86:971-976 (1995)). MGDF might be used to enhance the platelet donation process, since administration of MGDF increases circulating platelet counts to about three-fold the original value in healthy platelet donors.
TPO and MGDF exert their action through binding to the c-MP1 receptor which is expressed primarily on the surface of certain hematopoietic cells, such as megakaryocytes, platelets, CD34+ cells and primitive progenitor cells (Debili, N. et al., Blood 85:391-401 (1995); de Sauvage, F. J. et al, Nature 369:533-538 (1994); Bartley, T. D., et al., Cell 77:1117-1124 (1994); Lok, S. et al., Nature 369: 565-S (1994)). Like most receptors for interleukins and protein hormones, c-MP1 belongs to the class I cytokine receptor superfamily (Vigon, I. et al., Proc. Natl. Acad. Sci. USA 89:5640-5644 (1992)). Activation of this class of receptors involves ligand-binding induced receptor homodimerization which in turn triggers the cascade of signal transducing events.
In general, the interaction of a protein ligand with its receptor often takes place at a relatively large interface. However, as demonstrated in the case of human growth hormone bound to its receptor, only a few key residues at the interface actually contribute to most of the binding energy (Clackson, T. et al., Science 267:383-386 (1995)). This and the fact that the bulk of the remaining protein ligand serves only to display the binding epitopes in the right topology makes it possible to find active ligands of much smaller size.
In an effort toward this, the phage peptide library display system has emerged as a powerful technique in identifying small peptide mimetics of large protein ligands (Scott, J. K. et al., Science 249:386 (1990); Devlin, J. J. et al., Science 249:404 (1990)). By using this technique, small peptide molecules that act as agonists of the c-MP1 receptor were discovered (Cwirla, S. E. et al., Science 276:1696-1699 (1997)).
In such a study, random small peptide sequences displayed as fusions to the coat proteins of filamentous phage were affinity-eluted against the antibody-immobilized extracellular domain of c-MP1 and the retained phages were enriched for a second round of affinity purification. This binding selection and repropagation process was repeated many times to enrich the pool of tighter binders. As a result, two families of c-MP1-binding peptides, unrelated to each other in their sequences, were first identified. Mutagenesis libraries were then created to further optimize the best binders, which finally led to the isolation of a very active peptide with an IC50=2 nM and an EC50=400 nM (Cwirla, S. E. et al., Science 276:1696-1699 (1997)). This 14-residue peptide, designated as a TMP (for TPO Mimetic Peptide), has no apparent sequence homology to TPO or MGDF. The structure of this TMP compound is as follows:
Ile Glu Gly Pro Thr Leu Arg Gln Trp Leu Ala Ala Arg Alaxe2x80x83xe2x80x83SEQ ID NO: 1 
or
IEGPTLRQWLAARA using single letter amino acid abbreviations.
Previously, in a similar study on EPO mimetic peptides, an EPO mimetic peptide (EMP) was discovered using the same technique (Wrighton, N. C. et al., Science 273:458-463 (1996)), and was found to act as a dimer in binding to the EPO receptor (EPOR). The (ligand)2/(receptor)2 complex thus formed had a C2 symmetry according to X-ray crystallographic data (Livnah, O. et al., Science 273:464-471 (1996)). Based on this structural information, a covalently linked dimer of EMP in which the C-termini of two EMP monomers were crosslinked with a flexible spacer was designed and found to have greatly enhanced binding as well as in vitro/in vivo bioactivity (Wrighton, N. C., et al., Nature Biotechnology 15: 1261-1265 (1997)).
A similar C-terminal dimerization strategy was applied to the TPO mimetic peptide (TMP) (Cwirla, S. E. et al., Science 276:1696-1699 (1997)). It was found that a C-terminally linked dimer (Cxe2x80x94C link) of TMP had an improved binding affinity of 0.5 nM and a remarkably increased in vitro activity (EC50=0.1 nM) in cell proliferation assays (Cwirla, S. E. et al., Science 276:1696-1699 (1997)). The structure of this TMP Cxe2x80x94C dimer is shown below: 
In another aspect of the present invention, the tandem dimers may be further attached to one or more moieties that are derived from immunoglobulin proteins, referred to generally as the Fc region of such immunoglobulins. The resulting compounds are referred to as Fc fusions of TMP tandem dimers.
The following is a brief background section relating to the Fc regions of antibodies that are useful in connection with some of the present compounds.
Antibodies comprise two functionally independent parts, a variable domain known as xe2x80x9cFabxe2x80x9d, which binds antigen, and a constant domain, known as xe2x80x9cFcxe2x80x9d which provides the link to effector functions such as complement fixation or phagocytosis. The Fc portion of an immunoglobulin has a long plasma half-life, whereas the Fab is short-lived. (Capon, et al., Nature 337:525-531 (1989)).
Therapeutic protein products have been constructed using the Fc domain to attempt to provide longer half-life or to incorporate functions such as Fc receptor binding, protein A binding, complement fixation, and placental transfer which all reside in the Fc region of immunoglobulins (Capon, et al., Nature 337:525-531 (1989)). For example, the Fc region of an IgG1 antibody has been fused to CD30-L, a molecule which binds CD30 receptors expressed on Hodgkin""s Disease tumor cells, anaplastic lymphoma cells, T-cell leukemia cells and other malignant cell types. See, U.S. Pat. No. 5,480,981. IL-10, an anti-inflammatory and antirejection agent has been fused to murine Fcxcex32a in order to increase the cytokine""s short circulating half-life (Zheng, X. et al., The Journal of Immunology, 154; 5590-5600 (1995)). Studies have also evaluated the use of tumor necrosis factor receptor linked with the Fc protein of human IgG1 to treat patients with septic shock (Fisher, C. et al., N. Engl. J. Med., 334: 1697-1702 (1996); Van Zee, K. et al., The Journal of Immunology, 156: 2221-2230 (1996)). Fc has also been fused with CD4 receptor to produce a therapeutic protein for treatment of AIDS. See, Capon et al., Nature, 337:525-531 (1989). In addition, interleukin 2 has been fused to the Fc portion of IgG1 or IgG3 to overcome the short half life of interleukin 2 and its systemic toxicity. See, Harvill et al., Immunotechnology, 1: 95-105 (1995).
In spite of the availability of TPO and MGDF, there remains a need to provide additional compounds that have a biological activity of stimulating the production of platelets (thrombopoietic activity) and/or platelet precursor cells, especially megakaryocytes (megakaryopoietic activity). The present invention provides new compounds having such activity(ies), and related aspects.
The present invention provides a group of compounds that are capable of binding to and triggering a transmembrane signal through, i.e., activating, the c-MP1 receptor, which is the same receptor that mediates the activity of endogenous thrombopoietin (TPO). Thus, the inventive compounds have thrombopoietic activity, i.e., the ability to stimulate, in vivo and in vitro, the production of platelets, and/or megakaryocytopoietic activity, i.e., the ability to stimulate, in vivo and in vitro, the production of platelet precursors.
In a first preferred embodiment, the inventive compounds comprise the following general structure:
TMP1-(L1)n-TMP2 
wherein TMP1 and TMP2 are each independently selected from the group of compounds comprising the core structure:
X2-X3-X4-X5-X6-X7-X8-X9-X10,
wherein,
X2 is selected from the group consisting of Glu, Asp, Lys, and Val;
X3 is selected from the group consisting of Gly and Ala;
X4 is Pro;
X5 is selected from the group consisting of Thr and Ser;
X6 is selected from the group consisting of Leu, Ile, Val, Ala, and Phe;
X7 is selected from the group consisting of Arg and Lys;
X8 is selected from the group consisting of Gln, Asn, and Glu;
X9 is selected from the group consisting of Trp, Tyr, Cys, Ala, and Phe;
X10 is selected from the group consisting of Leu, Ile, Val, Ala, Phe, Met, and Lys;
L1 is a linker as described herein; and
n is 0 or 1;
and physiologically acceptable salts thereof.
In one embodiment, L1 comprises (Gly)n, wherein n is 1 through 20, and when n is greater than 1, up to half of the Gly residues may be substituted by another amino acid selected from the remaining 19 natural amino acids or a stereoisomer thereof.
In addition to the core structure X2-X10 set forth above for TMP1, and TMP2, other related structures are also possible wherein one or more of the following is added to the TMP1 and/or TMP2 core structure: X1 is attached to the N-terminus and/or X11, X12, X13, and/or X14 are attached to the C-terminus, wherein X1, X12, X13, and X14 are as follows:
X1 is selected from the group consisting of Ile, Ala, Val, Leu, Ser, and Arg;
X11 is selected from the group consisting of Ala, Ile, Val, Leu, Phe, Ser, Thr, Lys, His, and Glu;
X12 is selected from the group consisting of Ala, Ile, Val, Leu, Phe, Gly, Ser, and Gln;
X13 is selected from the group consisting of Arg, Lys, Thr, Val, Asn, Gln, and Gly; and
X14 is selected from the group consisting of Ala, Ile, Val, Leu, Phe, Thr, Arg, Glu, and Gly.
In a second preferred embodiment, the inventive compounds have the general formula:
(Fc)m-(L2)q-TMP1-(L1)n-TMP2-(L3)r-(Fc)p 
wherein TMP1, TMP2 and n are each as described above; L1, L2 and L3 are linker groups which are each independently selected from the linker groups described herein; Fc is an Fc region of an immunoglobulin (as defined herein below); m, p, q and r are each independently selected from the group consisting of 0 and 1, wherein at least one of m or p is 1, and further wherein if m is 0 then q is 0, and if p is 0, then r is 0; and physiologically acceptable salts thereof. In one embodiment, L1, L2, and L3 independently comprise (Gly)n, wherein n is 1 through 20, and when n is greater than 1, up to half of the Gly residues may be substituted by another amino acid selected from the remaining 19 natural amino acids or a stereoisomer thereof.
Derivatives of the above compounds (described below) are also encompassed by this invention.
The compounds of this invention are preferably peptides, and they may be prepared by standard synthetic methods or any other methods of preparing peptides. The compounds of this invention that encompass non-peptide portions may be synthesized by standard organic chemistry reactions, in addition to standard peptide chemistry reactions when applicable.
The compounds of this invention may be used for therapeutic or prophylactic purposes by incorporating them with appropriate pharmaceutical carrier materials and administering an effective amount to a subject, such as a human (or other mammal). Other related aspects are also included in the instant invention.